In recent years, there has been proposed lighting devices configured to control light output of semiconductor light emitting devices (i.e., adjust optical intensity of the semiconductor light emitting devices) and luminaires including the lighting devices (for example, see JP 2013-118133A, hereinafter referred to as “Document 1”).
As shown in FIG. 17, a lighting device 81 described in Document 1 includes a DC power supply circuit 82, a step-down chopper circuit 83, and a control circuit 84.
The step-down chopper circuit 83 includes a diode 85 and a switching device 86 connected in series between output ports of the DC power supply circuit 82, and an inductor 88 to be connected in series with a light source load 87 between two terminals (between an anode and a cathode) of the diode 85.
A resistor 89 for detecting a current (a chopper current) flowing through the switching device 86 is interposed between a source terminal of the switching device 86 and a negative electrode of the DC power supply circuit 82.
The control circuit 84 is formed of a control integrated circuit 90 and its peripheral components. A fourth pin P4 of the integrated circuit 90 is a terminal for detecting the above-described chopper current, and is electrically connected with a noise filter (not shown) that is formed of a resistor and a capacitor and is provided in the integrated circuit 90.
The control circuit 84 is configured to turn on the switching device 86 when the chopper current detected through the fourth pin P4 of the integrated circuit 90 becomes zero. The control circuit 84 is also configured to turn off the switching device 86 when the chopper current detected through the fourth pin P4 of the integrated circuit 90 reaches a first predetermined value.
In the lighting device 81, the control circuit 84 turns off the switching device 86 when the chopper current detected through the fourth pin P4 of the integrated circuit 90 reaches the first predetermined value. Accordingly, in the lighting device 81, an on-time period of the switching device 86 can be changed by changing the first predetermined value. The lighting device 81 can thereby control light output of the light source load 87.
However, in the lighting device 81, the fourth pin P4 of the integrated circuit 90 is electrically connected to the noise filter (an RC circuit) which is formed of the resistor and the capacitor provided in the integrated circuit 90. Accordingly, in the lighting device 81, a delay will possibly occur in turning off the switching device 86. That is, in the lighting device 81, the noise filter (the RC circuit) deforms a wave-shape of the chopper current (i.e., a delay time is added to the chopper current when the chopper current passes through the noise filter), and accordingly a delay will occur in turning off the switching device 86. In the lighting device 81, it is therefore difficult to precisely change the on-time period of the switching device 86. In addition, in the lighting device 81, it is difficult to shorten the on-time period of the switching device 86 to be smaller than the delay time of the noise filter (the RC circuit).